Presently, three kinds of positive electrode material are commonly used in lithium cell, such as lithium cobalt oxide, lithium nickel cobalt oxide and lithium manganese oxide. Lithium cobalt oxide and lithium nickel cobalt oxide are oxide with hexagonal system layered rock-salt structure and electrons in lithium ion migrating in the octahedral lamellar spacing formed by O—Co—O, possessing higher conductivity and reversibility of intercalation-deintercalation of lithium ion. Lithium manganese oxide is an oxide with spinel three-dimensional structure, and electrons in lithium ion migrating in the octahedral cubic channel formed by O—Mn—O, possessing higher conductivity and reversibility of intercalation-deintercalation of lithium ion as well. These positive electrode materials are widely used in the present lithium cell industry. However, metal cobalt is one of the rare elements on the earth and possesses radioactivity, and its oxide will violently react with electrolyte when the cell is over-charged or over-discharged, thereby releasing large quantity of heat which will cause the cell on fire until its explosion. Therefore, lithium cobalt oxide and lithium nickel cobalt oxide are of high manufacturing cost and low safety. Although lithium manganese oxide is cheaper and safer, its capacity is small, and its service life cycle under high temperature condition (above 55° C.) is short. The service life cycle of the lithium manganese oxide cell still fail to meet the practical demand even if it has undergone a doping and surface chemical treatment. Thus, in lithium cell industry, especially in high-power lithium cell, an environment-friendly and safe positive electrode material with low cost, large capacity is badly in need.
For this purpose, professor J. B. Goodenough etc. from University of Texas, USA (A. K. Padhi, K. S. Najundaswamy, C. Masgueslier, S. Okada and J. B. Goodenough, J. Eletrochem. Soc. 144, 1609-1613 (1997)) published an article in American Journal of Electrochemistry in 1997, disclosing a new lithium intercalating compound: lithium iron phosphate polycrystal LiFePO4. The polycrystal possesses reversibility of lithium ion intercalation-deintercalation, of which the lithium ion electrons migrate freely in FeO6 octahedral and PO4 tetrahedron structure. Theoretical discharging capacity of the lithium iron phosphate polycrystal can reach 170 mAh/g, when 1 molar of lithium ion is deintercalating from the structure. Owing to the abundant lithium and iron reserve, manufacturing cost of lithium iron phosphate is low. It is predicted herein that since the lithium iron phosphate material is provided with various characteristics such as environment-friendly, safe, low cost and high performance, it may have great prospect of application in cell industry.
However, the conductivity of lithium iron phosphate in room temperature is extra-low (10−9 S/cm), under normal discharging current (10−1 mA/cm2) condition, the actual capacity of lithium iron phosphate only accounts for 10% of the theoretical value (170 mAh/g). Thus, its application in cell is limited. In order to improve the conductivity of lithium iron phosphate, it is recently reported in article (Suag-Yoon Chang, Jason T. Bloking and Yetming Chiang, Nature, October 123-128(2002)) that after adding trace additives in the structure of lithium iron phosphate, such as Mg, Ti, Nb and Zr etc., the conductivity of lithium iron phosphate has been greatly improved in room temperature. However, the method of adding additives mentioned herein is complicated and the trace element is of high price, so it is not suitable for large-scale industrial production. Moreover, the room-temperature conductive space of lithium iron phosphate is larger, while its discharging voltage is lower, thereby affecting the energy density of the material.